package com.ufo.exercise;

import java.util.HashMap;
import java.util.Stack;
import java.util.concurrent.ConcurrentHashMap;

public class MStack {
    public static void main(String[] args) {
        HashMap m=new HashMap();
        m.put("x",null);
        ConcurrentHashMap m2=new ConcurrentHashMap();
        m2.put("x",null);
        Stack<Integer> stackJ=new Stack<>();
    }
}
